Just as a 411 operator won’t tell you an unlisted number, DNS can refuse to provide Internet addresses if it chooses. SOPA simply requires ISPs to delist the Internet addresses of foreign sites found by a US court to be dedicated to criminal activities. DNS has had the ability to delist sites since it was designed in 1987, and all widely used DNS services have this capability.
SOPA critics charge that such filtering breaks the Internet, but it does no such thing as long as it’s done sensibly. (Security experts criticized an early version of SOPA, but the amended bill addresses their concerns.) It’s a practical means of protecting consumers from rogue sites that traffic in illegal goods.